Spending on agency staff to cover NHS vacancies has risen markedly over the last few years in Wales, according to a financial watchdog.
Spending on agency staff had risen 171% over seven years, peaking at £164.4m in 2016-17, the auditor general Adrian Crompton said.
He called for strong leadership to manage the situation.
It comes in the wake of health boards like Hywel Dda facing financial problems, underpinned by vacancies.
The Welsh Government said spending on agency staff and locums was £30m less in the last year after controls and management processes were introduced.
Mr Crompton sets out a "facts only report" to better understand the causes of the spending on agencies.
What are the main issues?
- 82% of agency spend so far in 2018-19 is on covering for vacancies, with NHS Wales looking to both cut the demand for agency staff and the price it pays for them.
- Skill shortages, increases in demand and levels of sickness absence are other factors.
- Temporary staff for the health service generally cost more - and those supplied by agencies tend to be the most expensive source of temporary workers.
- Two thirds go on doctors and dentists, and another third on nurses and midwives.
Particular problems in recruitment faced in north and west Wales can be seen in the amount spent on agency staff.
Hywel Dda alone was spending £1.2m a month in 2013-14 before it rose to £3.7m a month by the end of 2015.
The watchdog said that despite each health board holding data on the agency staff they use, there was still no all-Wales analysis of how many were being hired through agencies or their specialties and grades.
Mr Crompton said NHS Wales was now developing arrangements at an all-Wales level to better understand agency usage - as well as addressing recruitment issues.
It was also working to reduce the rates of agency pay, which had been driven up by demand and fuelled by competition between health bodies.
By Owain Clarke, BBC Wales health correspondent
The NHS always has - and always will - need to look at using temporary staff.
They help to plug gaps, and let's face it, keep patients safe if permanent staff are sick or if a health board is having trouble filling vacancies.
But there is concern that the NHS has become too reliant on agency workers day-to-day, just to keep the show on the road.
It's not a problem unique to Wales; the NHS in all four UK nations spends too much on agency staff.
And any health body would prefer to spend its cash on permanent staff rather than paying more for people to fill-in.
But it's not always easy.
Smaller hospitals or surgeries in rural areas might find it more difficult to attract recruits - particularly younger doctors and there are big shortages in some specialist areas.
Meanwhile some experts argue that this is a symptom of poor workforce planning - with the NHS too preoccupied on fixing problems of today rather than preparing for tomorrow.
Mr Crompton said: "I hope it [the report] will be used by a wide range of people and professionals in Wales, along with the data tool we have produced, to help NHS Wales continue to bring down and control these costs at a time of significant financial pressure."
A Welsh Government spokesman added: "We welcome this report, which will inform future activity in strengthening leadership to steer this work, delivering future efficiencies and developing a single source of consistent data collection."